Plastics are finding increasing use in manufactured goods. For example, certain automobiles have plastic body panels, and aircraft have plastic interior paneling and exterior skin panels formed of plastics and plastic composites. While plastics offer several excellent properties including low weight, formability and low cost, plastics also have significant disadvantages. In general, plastic surfaces are not as hard or abrasion resistant as metal surfaces. Furthermore, while some plastics may be transparent, glass, which is much heavier and more expensive, remains the material of choice in certain critical applications such as automobiles and passenger aircraft windshields. Substituting polymeric materials, such as acrylic or polycarbonate materials, may lead to lighter transparencies, but may also enable re-designing the overall shape of the cockpit. Currently, stretched acrylic materials are used to fabricate aircraft passenger windows. Acrylic materials are used because of flexibility, lightness, and easy formability. However, acrylic materials are soft material and can be easily scratched. Water absorption, chemical attack, and mechanically induced scratches may lead to crazing when stress is applied, for example when the acrylic material experiences operational stresses as in an aircraft passenger window application.
Industry wide, polymer based transparencies are protected against wear and other chemical/nature induced degradation through siloxane-based coatings. At the present time, polycarbonate and other types of polymeric windows are protected by sol-gel based polysiloxane coatings. The term sol-gel or solution-gelation refers to materials undergoing a series of reactions like hydrolization and condensation. The sol-gel coatings are homogeneous mixtures of a solvent, an organosilane, an alkoxide and a catalyst that are processed to form a suitable coating. The sol-gel coatings provide high transmittance and limited durability against wear and UV induced degradation. Typically, a metal alkoxide or metal salt is hydrolyzed to form a metal hydroxide. The metal hydroxide then condenses in solution to form a hybrid organic/inorganic polymer. The ratio of organic to inorganic components in the polymer matrix is controlled to maximize the performance for a given application. For example, increasing the organic groups would improve flexibility but may compromise wear and environmentally induced durability. The sol-gel coating may include materials such as cerium or titanium to improve abrasion resistance and ultraviolet induced degradation of the coatings. A typical application process would consist of component surface cleaning, followed by the application of the coating via a flow, spray or dip process. The surface cleaning may be achieved by solvent wiping with, for example, isopropyl alcohol, or by exposing the component to oxygen plasma. The sol-gel coatings can be cured at room temperature or elevated temperatures. For example, stretched acrylics must be cured at temperatures less than 180° F.
The coatings used at the present time exhibit only a moderate durability. There is a need for a transparent, hard coating with excellent durability that would improve component lifetime. The coating should provide improved resilience against chemicals commonly encountered in product maintenance and also excellent weatherability characteristics. The coating should be both hard and flexible, so that it tolerates the flexing of the polymeric material due to operation and thermal stresses. The coating should be provided by a simple process and at a low cost.
Commercial Passenger aircraft cockpit windows are currently made of multi pane glass because of its strength and abrasion resistance. Efforts are ongoing to switch to polymeric material based flight deck windows as these materials are light, and amenable to forming desired shapes at a low cost. While plastics offer several excellent properties such as light weight, formability, and low cost, plastics also have significant short comings. In general, plastic surfaces are not as hard or abrasion resistant as glass or steel surfaces. Polymeric materials are susceptible to particle (e.g., sand)/water induced erosion and chemical crazing; protective hard coatings are needed to maintain the optical quality of the windows in use.
In addition, while a polymeric-glass laminate has less weight over an all-glass laminate, additional weight reduction could be achieved by removal of the glass facing ply if erosion and abrasion were not a problem. The polymeric-glass laminate also suffers from thermally induced stresses from the thermal expansion difference between glass and polymeric layers, which reduces service life. Furthermore, matching contours between glass and polymeric plies poses manufacturing problems, and can lead to optical and service related issues in the final part.
Duplex coating schemes have been developed that offer improved performance. These duplex coating schemes have been specifically designed for applications where good visibility is required but not critical at all times, such as aircraft passenger windows. However, the optical quality of cockpit windows is a flight critical property. With current coatings there is no non-destructive way to determine if the transparent coating is still present and protecting the polymeric substrate. Therefore, a method and coating is needed to continually insure the integrity of a protective coating.
Furthermore, there is a need for an improved wear resistant coating for polymeric transparencies and a method to insure that optical clarity is maintained. The present disclosure fulfills these needs, and further provides related advantages.